Systems and methods for lp-egr delivery in a variable displacement engine

ABSTRACT

Methods and systems are provided for controlling LP-EGR flow in a variable displacement engine. In one example, a method may include providing a higher fixed LP-EGR percentage relative to a total intake air flow during a VDE mode, and providing a lower fixed EGR percentage relative to the total intake air flow during a non-VDE mode. Further, during a tip-out when operating in the VDE mode at a load below a threshold, transitioning out of the VDE mode after transitioning the LP-EGR from the higher fixed percentage to the lower fixed percentage.

FIELD

The present description relates generally to methods and systems forimproving LP-EGR delivery in a variable displacement engine.

BACKGROUND/SUMMARY

Exhaust gas recirculation (EGR) systems recirculate a portion of exhaustgas from an engine exhaust to an engine intake system to improve fueleconomy and vehicle emissions by reducing throttling losses andcombustion temperatures. In turbocharged engines, an EGR system mayinclude a low-pressure EGR (LP-EGR) circuit that diverts exhaust gasesfrom downstream of a turbine of a turbocharger and injects the gasesbefore a compressor. However, the LP-EGR circuit has a long transportdelay, as the exhaust gases must travel through the turbochargercompressor, high pressure air induction plumbing, charge air cooler, andintake manifold before reaching the combustion chamber. As a result, itmay be difficult to provide a desired amount of EGR to the cylinders,particularly during transient conditions.

One example approach for managing the long transport delay is shown byStyles et al. in US 20120023937. Therein, the LP-EGR system is operatedat a fixed EGR percentage rate of fresh air flow across an area of aspeed-load map, including a minimum engine load in order to improvetransient control of LP-EGR (e.g., during a driver tip-out event whenminimum load may be encountered).

However, the inventors herein have recognized issues with the aboveapproach. Specifically, in an engine configured for variable selectivecylinder deactivation, when transitioning between operating modes, dueto differences in EGR tolerance, engine misfires can occur. As such, thefixed EGR percentage is based on an EGR tolerance level during theminimum engine load. Herein, engine load, or load, will be used todescribe the overall engine air flow or engine torque. Cylinder loadwill be used to describe an average air flow per active cylinder in theengine. In this way, cylinder load will increase at the same engine loadwhen cylinders have been selectively deactivated. However, in enginesconfigured for selective cylinder deactivation (e.g., variabledisplacement engines (VDE)), the EGR tolerance level of the engine mayvary based on whether the engine is operating with all cylinders activeor with one or more cylinders deactivated. For example, when operatingin a VDE mode with one or more cylinders deactivated, due to anincreased cylinder load on the remaining active cylinders, a minimumcylinder load encountered in the VDE mode may be greater than theminimum cylinder load during a non-VDE mode when all the cylinders areactive and combusting. Consequently, the engine may tolerate higher EGRlevels when operating in the VDE mode than when operating in the non-VDEmode. Therefore, if the fixed EGR percentage is based on engineoperation in the VDE mode, the EGR percentage may be greater thanrequired for the non-VDE mode. As a result, there may be excess dilutionof intake air in the non-VDE mode which may increase combustionstability issues and the propensity for engine misfires.

Further, during a tip-out occurring when operating in the VDE mode withthe fixed EGR percentage based on the VDE minimum load, due to largetransport delays associated with the LP-EGR system, the engine operationmay transition out of the VDE mode (in order to reduce Noise Vibrationand Harshness (NVH) issues, for example) before the EGR is purged fromthe air induction system. As a result, the engine may be exposed tohigher EGR levels than is tolerable by the engine in the non-VDE mode,leading to increased combustion instability and misfires.

In one example, some of the above issues can be at least partlyaddressed by a method for an engine comprising: in response to a tip-outoccurring while operating the engine below a threshold engine load withone or more cylinders deactivated and with LP-EGR provided at a higherfixed schedule relative to intake air flow, delaying reactivation of thedeactivated cylinders until EGR has reduced from the higher fixedschedule to a lower fixed schedule relative to intake air flow. In thisway, cylinder reactivation may be adjusted based on cylinder EGRtolerance to reduce occurrence of misfire events.

As an example, a VDE engine system may include a LP-EGR system forproviding EGR. When operating the VDE engine in a VDE mode with one ormore cylinders deactivated and in a speed-load range for fixed EGRpercentage, EGR may be provided at a higher fixed EGR percentage due tothe higher EGR tolerance of the remaining active cylinders that areoperating at a higher average cylinder load. In comparison, whenoperating in a non-VDE mode with all the cylinders active, EGR may beprovided at a lower fixed percentage due to the lower EGR tolerance ofengine cylinders operating at a lower average cylinder load. Further,during a tip-out condition occurring when operating in the VDE mode, itmay be desirable to transition out of the VDE mode in order to reduceNVH issues or due to hardware constraints, for example. During suchtip-out conditions when engine transition from VDE mode to non-VDE modeis desired, if an engine load is below a threshold engine load,transition of engine operation out of the VDE mode may be delayed untilthe EGR percentage relative to intake air flow decreases to the lowerfixed percentage. This avoids the condition where the engine isoperating with a higher EGR schedule than the engine can tolerate.However, if the engine is operating in the VDE mode at an engine loadabove the threshold load, the engine may be allowed to transition out ofthe VDE mode while transitioning EGR from the higher fixed percentage tothe lower fixed percentage.

In this way, by providing EGR in two different fixed schedules includingthe higher fixed percentage of EGR during the VDE mode and the lowerfixed percentage of EGR during the non-VDE mode, during each mode, theengine may be operated with an EGR schedule that the engine cantolerate, while allowing the EGR schedule to be varied based on thedifferent EGR tolerances in the different modes. By allowing a higherflat EGR schedule to be applied when selected cylinders are deactivated,the EGR percentage in the air induction system can be raised, increasingthe effectiveness of EGR. By allowing a lower flat EGR schedule to beapplied during the non-VDE mode, excess intake air dilution with EGR maybe reduced, reducing slow burn issues. Further, by delaying enginetransition out of the VDE mode responsive to a tip-out until the EGRlevel corresponding to the VDE mode is cleared out of the air inductionsystem, engine misfires and slow combustion issues due to engineexposure to higher than tolerable EGR levels is reduced. Consequently,engine misfire events are reduced and combustion stability may beimproved.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a partial engine view.

FIG. 2 shows a schematic diagram of an engine system with dual cylinderbanks, the engine including an exhaust gas recirculation system.

FIG. 3 shows a high level flow chart of an example method fordetermining an LP-EGR schedule.

FIG. 4 shows a high level flow chart depicting an example method foradjusting a fixed mode LP-EGR schedule during engine operation based onindividual cylinder deactivation.

FIG. 5A shows an example speed-load map for a variable LP-EGR scheduleand a fixed LP-EGR schedule.

FIG. 5B shows an example speed-load map of distinct fixed EGR schedulesincluding a first fixed LP-EGR schedule that is applied in the presenceof cylinder deactivation and a second fixed LP-EGR schedule that isapplied in the absence of cylinder deactivation.

FIG. 6 shows an example adjusting of fixed LP-EGR schedules duringtransition between engine operation in VDE and non-VDE modes, accordingto embodiments of the present disclosure.

DETAILED DESCRIPTION

The present description relates to an EGR system coupled to aturbocharged variable displacement engine in a motor vehicle. In onenon-limiting example, the VDE engine may be configured as part of theengine system illustrated at FIG. 1, wherein the engine includes atleast one cylinder, a control system, a turbocharger, and an exhaust gasrecirculation system, among other features. The engine may also beconfigured with a plurality of cylinder banks as illustrated at FIG. 2.An engine controller may be configured to perform a control routine,such as the example routines of FIGS. 3-4 to adjust the LP-EGR schedulebased on minimum cylinder load requirements of the engine as the enginetransitions between operating with all cylinders active (non-VDE mode)and operating with one or more deactivated cylinders (VDE mode). Thevarious EGR schedules may be selected based on engine-speed load maps,such as the speed-load maps of FIGS. 5A and 5B. An example EGR scheduleadjustment is illustrated with reference to FIG. 6.

Referring now to FIG. 1, it shows a schematic diagram of one cylinder ofmulti-cylinder engine 10, which may be included in a propulsion systemof an automobile, is shown. Engine 10 may be controlled at leastpartially by a control system including controller 12 and by input froma vehicle operator 132 via an input device 130. In this example, inputdevice 130 includes an accelerator pedal and a pedal position sensor 134for generating a proportional pedal position signal PP. Combustionchamber (i.e., cylinder) 30 of engine 10 may include combustion chamberwalls 32 with piston 36 positioned therein. In some embodiments, theface of piston 36 inside cylinder 30 may have a bowl. Piston 36 may becoupled to crankshaft 40 so that reciprocating motion of the piston istranslated into rotational motion of the crankshaft. Crankshaft 40 maybe coupled to at least one drive wheel of a vehicle via an intermediatetransmission system. Further, a starter motor may be coupled tocrankshaft 40 via a flywheel to enable a starting operation of engine10.

Combustion chamber 30 may receive intake air from intake manifold 44 viaintake passage 42 and may exhaust combustion gases via exhaust passage48. Intake manifold 44 and exhaust passage 48 can selectivelycommunicate with combustion chamber 30 via respective intake valve 52and exhaust valve 54. In some embodiments, combustion chamber 30 mayinclude two or more intake valves and/or two or more exhaust valves.

Intake valve 52 may be controlled by controller 12 via electric valveactuator (EVA) 51. Similarly, exhaust valve 54 may be controlled bycontroller 12 via EVA 53. Alternatively, the variable valve actuator maybe electro hydraulic or any other conceivable mechanism to enable valveactuation. During some conditions, controller 12 may vary the signalsprovided to actuators 51 and 53 to control the opening and closing ofthe respective intake and exhaust valves. The position of intake valve52 and exhaust valve 54 may be determined by valve position sensors 55and 57, respectively. In alternative embodiments, one or more of theintake and exhaust valves may be actuated by one or more cams, and mayutilize one or more of cam profile switching (CPS), variable cam timing(VCT), variable valve timing (VVT) and/or variable valve lift (VVL)systems to vary valve operation. For example, cylinder 30 mayalternatively include an intake valve controlled via electric valveactuation and an exhaust valve controlled via cam actuation includingCPS and/or VCT.

Fuel injector 66 is shown coupled directly to combustion chamber 30 forinjecting fuel directly therein in proportion to the pulse width ofsignal FPW received from controller 12 via electronic driver 68. In thismanner, fuel injector 66 provides what is known as direct injection offuel into combustion chamber 30. The fuel injector may be mounted in theside of the combustion chamber or in the top of the combustion chamber,for example. Fuel may be delivered to fuel injector 66 by a fuel system(not shown) including a fuel tank, a fuel pump, and a fuel rail.

Ignition system 88 can provide an ignition spark to combustion chamber30 via spark plug 92 in response to spark advance signal SA fromcontroller 12, under select operating modes. Though spark ignitioncomponents are shown, in some embodiments, combustion chamber 30 or oneor more other combustion chambers of engine 10 may be operated in acompression ignition mode, with or without an ignition spark.

Intake passage 42 may include throttles 62 and 63 having throttle plates64 and 65, respectively. In this particular example, the positions ofthrottle plates 64 and 65 may be varied by controller 12 via signalsprovided to an electric motor or actuator included with throttles 62 and63, a configuration that is commonly referred to as electronic throttlecontrol (ETC). In this manner, throttles 62 and 63 may be operated tovary the intake air provided to combustion chamber 30 among other enginecylinders. The positions of throttle plates 64 and 65 may be provided tocontroller 12 by throttle position signals TP. Pressure, temperature,and mass air flow may be measured at various points along intake passage42 and intake manifold 44. For example, intake passage 42 may include amass air flow sensor 120 for measuring clean air mass flow enteringthrough throttle 63. The clean air mass flow may be communicated tocontroller 12 via the MAF signal.

Engine 10 may further include a compression device such as aturbocharger or supercharger including at least a compressor 162arranged upstream of intake manifold 44. For a turbocharger, compressor162 may be at least partially driven by a turbine 164 (e.g., via ashaft) arranged along exhaust passage 48. For a supercharger, compressor162 may be at least partially driven by the engine and/or an electricmachine, and may not include a turbine. Thus, the amount of compressionprovided to one or more cylinders of the engine via a turbocharger orsupercharger may be varied by controller 12. A charge air cooler 154 maybe included downstream from compressor 162 and upstream of intake valve52. Charge air cooler 154 may be configured to cool gases that have beenheated by compression via compressor 162, for example. In oneembodiment, charge air cooler 154 may be upstream of throttle 62.Pressure, temperature, and mass air flow may be measured downstream ofcompressor 162, such as with sensor 145 or 147. The measured results maybe communicated to controller 12 from sensors 145 and 147 via signals148 and 149, respectively. Pressure and temperature may be measuredupstream of compressor 162, such as with sensor 153, and communicated tocontroller 12 via signal 155.

Further, in the disclosed embodiments, an EGR system may route a desiredportion of exhaust gas from exhaust passage 48 to intake manifold 44.FIG. 1 shows an HP-EGR system and an LP-EGR system, but an alternativeembodiment may include only an LP-EGR system. The HP-EGR is routedthrough HP-EGR passage 140 from upstream of turbine 164 to downstream ofcompressor 162. The amount of HP-EGR provided to intake manifold 44 maybe varied by controller 12 via HP-EGR valve 142. The LP-EGR is routedthrough LP-EGR passage 150 from downstream of turbine 164 to upstream ofcompressor 162. The amount of LP-EGR provided to intake manifold 44 maybe varied by controller 12 via LP-EGR valve 152. The HP-EGR system mayinclude HP-EGR cooler 146 and the LP-EGR system may include LP-EGRcooler 158 to reject heat from the EGR gases to engine coolant, forexample.

Under some conditions, the EGR system may be used to regulate thetemperature of the air and fuel mixture within combustion chamber 30.Thus, it may be desirable to measure or estimate the EGR mass flow. EGRsensors may be arranged within EGR passages and may provide anindication of one or more of mass flow, pressure, and temperature of theexhaust gas. For example, an HP-EGR sensor 144 may be arranged withinHP-EGR passage 140.

In some embodiments, one or more sensors may be positioned within LP-EGRpassage 150 to provide an indication of one or more of a mass flow,pressure, temperature, and air-fuel ratio of exhaust gas recirculatedthrough the LP-EGR passage. Exhaust gas diverted through LP-EGR passage150 may be diluted with fresh intake air at a mixing point located atthe junction of LP-EGR passage 150 and intake passage 42. Specifically,by adjusting LP-EGR valve 152 in coordination with first air intakethrottle 63 (positioned in the air intake passage of the engine intake,upstream of the compressor), a dilution of the EGR flow may be adjusted.

A percent dilution of the LP-EGR flow may be inferred from the output ofa sensor 145 in the engine intake gas stream. Specifically, sensor 145may be positioned downstream of first intake throttle 63, downstream ofLP-EGR valve 152, and upstream of second main intake throttle 62, suchthat the LP-EGR dilution at or close to the main intake throttle may beaccurately determined. Sensor 145 may be, for example, an oxygen sensorsuch as a UEGO sensor.

Exhaust gas sensor 126 is shown coupled to exhaust passage 48 downstreamof turbine 164. Sensor 126 may be any suitable sensor for providing anindication of exhaust gas air/fuel ratio such as a linear oxygen sensoror UEGO (universal or wide-range exhaust gas oxygen), a two-state oxygensensor or EGO, a HEGO (heated EGO), a NO_(X), HC, or CO sensor.

Emission control devices 71 and 72 are shown arranged along exhaustpassage 48 downstream of exhaust gas sensor 126. Devices 71 and 72 maybe a selective catalytic reduction (SCR) system, three way catalyst(TWC), NO_(X) trap, various other emission control devices, orcombinations thereof. For example, device 71 may be a TWC and device 72may be a particulate filter (PF). In some embodiments, PF 72 may belocated downstream of TWC 71 (as shown in FIG. 1), while in otherembodiments, PF 72 may be positioned upstream of TWC 72 (not shown inFIG. 1).

Controller 12 is shown in FIG. 1 as a microcomputer, includingmicroprocessor unit 102, input/output ports 104, an electronic storagemedium for executable programs and calibration values shown as read onlymemory chip 106 in this particular example, random access memory 108,keep alive memory 110, and a data bus. Controller 12 may receive varioussignals from sensors coupled to engine 10, in addition to those signalspreviously discussed, including measurement of inducted mass air flow(MAF) from mass air flow sensor 120; engine coolant temperature (ECT)from temperature sensor 112 coupled to cooling sleeve 114; a profileignition pickup signal (PIP) from Hall effect sensor 118 (or other type)coupled to crankshaft 40; throttle position (TP) from a throttleposition sensor; and absolute manifold pressure signal, MAP, from sensor122. Engine speed signal, RPM, may be generated by controller 12 fromsignal PIP. Manifold pressure signal MAP from a manifold pressure sensormay be used to provide an indication of vacuum, or pressure, in theintake manifold. Note that various combinations of the above sensors maybe used, such as a MAF sensor without a MAP sensor, or vice versa.During stoichiometric operation, the MAP sensor can give an indicationof engine torque. Further, this sensor, along with the detected enginespeed, can provide an estimate of charge (including air) inducted intothe cylinder. In one example, sensor 118, which is also used as anengine speed sensor, may produce a predetermined number of equallyspaced pulses every revolution of the crankshaft.

Storage medium read-only memory 106 can be programmed with computerreadable data representing instructions executable by processor 102 forperforming the methods described below as well as other variants thatare anticipated but not specifically listed.

As described above, FIG. 1 shows only one cylinder of a multi-cylinderengine, and that each cylinder may similarly include its own set ofintake/exhaust valves, fuel injector, spark plug, etc. In FIG. 2, anexample of an engine system including a plurality of cylinder banks andan exhaust gas recirculation system is illustrated. In one embodiment,engine 10 may comprise a turbocharger including compressor 162 andturbine 164, throttle 63 upstream of compressor 162, and a low-pressureexhaust gas recirculation (LP-EGR) system. The LP-EGR system may routeEGR from downstream of turbine 164 to upstream of compressor 162 anddownstream of throttle 63. The engine system may further comprise aHP-EGR system which routes EGR from upstream of turbine 164 todownstream of throttle 62.

Turning to FIG. 2, air may enter engine 10 through an air filter 210.Air filter 210 may be configured to remove solid particulates from theair so a clean air mass may enter engine 10. The clean air mass flow maybe measured as it flows past mass air flow sensor 120 and then throughintake throttle 63. The clean air mass flow measured by mass air flowsensor 120 may be communicated to controller 12. In one embodiment, theclean air mass may be split between the different cylinder banks ofengine 10 downstream of intake throttle 63 and upstream of turbochargercompressor 162. An EGR system may inject exhaust gas upstream ofturbocharger compressor 162 so that a combination of clean air andexhaust gas can be compressed by turbocharger compressor 162. In oneembodiment, turbocharger compressor 162 may include a first compressor162 a for a first cylinder bank and a second compressor 162 b for asecond cylinder bank.

In one example, engine 10 may be a V8 engine with the first and secondbanks each having four cylinders. As one non-limiting example, engine 10can be included as part of a propulsion system for a passenger vehicle.

Engine 10 may have cylinders (depicted in FIG. (1) with selectivelydeactivatable intake valves and selectively deactivatable exhaustvalves. In one example, the intake valves and exhaust valves may beconfigured for electric valve actuation (EVA) via electric individualcylinder valve actuators. While the example of FIG. 1 shows eachcylinder having a single intake valve and a single exhaust valve, inalternate examples, each cylinder may have a plurality of selectivelydeactivatable intake valves and/or a plurality of selectivelydeactivatable exhaust valves.

During selected conditions, such as when the full torque capability ofthe engine is not needed, one or more cylinders of engine 10 may beselected for selective deactivation (herein also referred to asindividual cylinder deactivation). This may include selectivelydeactivating one or more cylinders on only the first bank, one or morecylinders on only the second bank, or one or more cylinders on each ofthe first and second bank. The number and identity of cylindersdeactivated on each bank may be symmetrical or asymmetrical.

During the deactivation, selected cylinders may be deactivated byclosing the individual cylinder valve mechanisms, such as intake valvemechanisms, exhaust valve mechanisms, or a combination of both. Cylindervalves may be selectively deactivated via hydraulically actuated lifters(e.g., lifters coupled to valve pushrods), via a cam profile switchingmechanism in which a cam lobe with no lift is used for deactivatedvalves, or via the electrically actuated cylinder valve mechanismscoupled to each cylinder. In addition, fuel flow and spark to thedeactivated cylinders may be stopped, such as by deactivating cylinderfuel injectors.

In some examples, engine 10 may have selectively deactivatable (direct)fuel injectors and the selected cylinders may be deactivated by shuttingoff the respective fuel injectors while maintaining operation of theintake and exhaust valves such that air may continue to be pumpedthrough the cylinders.

While the selected cylinders are disabled, the remaining enabled oractive cylinders continue to carry out combustion with fuel injectorsand cylinder valve mechanisms active and operating. To meet the torquerequirements, the engine produces the same amount of torque on theactive cylinders. As a result, the remaining active cylinders operatewith a higher average cylinder load. This requires higher manifoldpressures, resulting in lowered pumping losses and increased engineefficiency. Also, the lower effective surface area (from only theenabled cylinders) exposed to combustion reduces engine heat losses,improving the thermal efficiency of the engine.

Cylinders may also be deactivated to provide a specific firing patternbased on a designated control algorithm. More specifically, selected“skipped” working cycles may not be fired while other “active” workingcycles are fired. Optionally, a spark timing associated with a selectedfiring of a selected working chamber may also be adjusted based on afiring order or firing history of the selected working chamber. Theengine controller 12 may be configured with suitable logic fordetermining a cylinder deactivation (or skip-firing) pattern based onengine operating conditions.

The compressed combination of clean air and exhaust gas downstream ofturbocharger compressor 162 may be cooled by a charge air cooler (CAC)154 upstream of a second throttle 62. In one embodiment, the oxygencontent of the airflow downstream from turbocharger compressor 162 maybe measured by a sensor 145 upstream of CAC 154. In an alternateembodiment, the oxygen content of the airflow downstream fromturbocharger compressor 162 may be measured by a sensor 147 downstreamof CAC 154. Measurements from sensors 145 and/or 147 may be communicatedto controller 12.

In one embodiment, high pressure exhaust gas may be combined with thecompressed combination of clean air and exhaust gas downstream ofthrottle 62 and upstream of intake manifold 44. The combination of gasesmay be routed to one or more cylinder banks by intake manifold 44. Aftercombustion in the cylinders, exhaust gas may be routed through exhaustpassage 48. In one embodiment, exhaust passage 48 includes an exhaustmanifold for each bank of cylinders, such as exhaust manifold 48 a for afirst cylinder bank and exhaust manifold 48 b for a second cylinderbank.

At least a portion of the exhaust gasses may drive a turbine 164 of theturbocharger. In one embodiment, turbine 164 may include a first turbine164 a for a first cylinder bank and a second turbine 164 b for a secondcylinder bank. In one embodiment, at least a portion of the exhaustgasses may be routed through an HP-EGR system. For example, an HP-EGRsystem may include HP-EGR cooler 146 and valve 142 for routing cooledexhaust gasses upstream of intake manifold 44. In one embodiment, aHP-EGR system may include a first HP-EGR cooler 146 a and valve 142 afor a first cylinder bank and a second HP-EGR cooler 146 b and valve 142b for a second cylinder bank.

Downstream from turbine 164, at least a portion of the exhaust gassesmay flow downstream through emission control device 71 and muffler 220.In one embodiment, emission control device 71 may include a firstlight-off catalyst 71 a for a first cylinder bank and a second light-offcatalyst 71 b for a second cylinder bank. Muffler 220 may be configuredto dampen exhaust noise from engine 10.

At least a portion of the exhaust gasses from downstream of turbine 164may be routed upstream of turbocharger compressor 162 by an LP-EGRsystem. For example, an LP-EGR system may include LP-EGR cooler 158 andvalve 152 for routing cooled exhaust gasses upstream of compressor 162.In one embodiment, an LP-EGR system may include a first LP-EGR cooler158 a and valve 152 a for a first cylinder bank and a second LP-EGRcooler 158 b and valve 152 b for a second cylinder bank.

Thus, engine 10 may comprise both an HP-EGR and an LP-EGR system toroute exhaust gases back to the intake. In some embodiments, the LP-EGRsystem may be controlled to operate under various schedules aselaborated further with respect to FIGS. 3-6 based on engine operatingparameters.

In one example, the LP-EGR system may be controlled to provide a fixedpercentage of LP-EGR with respect to a total intake air flow whenoperating in a fixed schedule speed-load range. For example, even as thespeed and engine load conditions vary, as long as the operatingconditions fall within the fixed schedule speed-load range, thepercentage of LP-EGR with respect to intake air may remain fixed.Further, when operating in the fixed range, if the engine is in a VDEmode, a higher fixed percentage of LP-EGR may be provided due to thehigher minimum cylinder load and higher EGR tolerance of activecylinders when the engine is operating with one or more cylindersdeactivated, while a lower fixed percentage of LP-EGR may be provideddue to the lower minimum cylinder load and lower EGR tolerance ofcylinders when the engine is operating with all cylinders active.

In this way, by utilizing the higher LP-EGR percentage for the VDE modeand the lower LP-EGR percentage for the non-VDE, the technical effect ofdelivering EGR that better matches the cylinder's EGR tolerance isprovided. In addition, excess intake air dilution with EGR is reduced,improving combustion stability.

Further, while the engine is operating in the VDE mode with a higherflat LP-EGR schedule, in response to a torque transient (such as atip-out, for example) occurring at an engine load below a thresholdengine load, and in response to leave the VDE mode (in order to reduceNVH issues during the tip-out, for example), the higher EGR percentagemay be reduced to the lower EGR percentage before transitioning theengine out of the VDE mode. As such, there may be delays in purgingLP-EGR from the air induction system due to associated transport delays.Consequently, if EGR is not purged to lower levels before reactivatingengine cylinders, the cylinders may be exposed to a higher EGR levelthan they can tolerate. As a result, combustion stability may beimproved and a tendency for engine misfires and slow burns may bereduced. Details of LP-EGR scheduling during different modes of engineoperation will be further elaborated with respect to FIGS. 3-6.

FIG. 3 is a flow chart illustrating a method 300 for determining anLP-EGR schedule based on engine operating conditions. The method of FIG.3 may be stored as executable instructions in non-transitory memory ofcontroller 12 shown in FIGS. 1-2 and carried out by the controller incombination with the various sensors, actuators, and engine componentsillustrated in FIGS. 1-2.

Method 300 may include, at 302, determining engine operating conditions.Engine operating conditions such as engine speed, engine load, vehiclespeed, engine temperature, etc., may be measured and/or estimated fromsensors including throttle position sensor, pedal position sensor, etc.Method 300 may then determine whether EGR is to be enabled at 304, basedon the engine operating parameters determined at 302. EGR may be enabledat low engine speed-load conditions. EGR may be disabled when enginetemperature is below a threshold, for example, or when the engine hasbeen at idle for an extended period of time.

If it is determined that EGR is not be enabled, method 300 returns. Ifit is determined that EGR is to be enabled, method 300 proceeds to 306to determine an HP-EGR schedule (including an HP-EGR rate, percentage,amount, temperature, etc.) based on engine speed and load conditions.The amount of HP-EGR delivered to the intake may be based on anengine-speed load map stored in the memory of controller 12. Method 300may then proceed to 308 to determine an LP-EGR schedule (including anLP-EGR rate, percentage, amount, temperature, etc.) based on theoperating conditions determined at 302. In some embodiments, the LP-EGRschedule may be determined based on a speed-load table stored in thememory of controller 12. In addition, the LP-EGR schedule may beadjusted based on the determined HP-EGR schedule to provide an overallengine dilution. One example of an engine speed-load map depicting twoLP-EGR operating schedules, fixed and variable, is shown at FIG. 5A.

At 310, it may be determined whether engine speed and engine load are inthe fixed schedule range. In one embodiment, the fixed schedule rangecomprises all engine loads from mid load down to minimum engine load,and/or engine speeds lower than a threshold, such as 3500 RPM. Minimumengine load as described herein comprises the lowest possible engineload allowable for current operating conditions, e.g. the lowest engineload that sustains combustion for current engine speed, temperature,etc., and may correspond to a closed throttle engine load for currentengine speed conditions. In some conditions, the minimum engine load maybe lower than the engine load at idle. Thus, the minimum engine load maybe encountered during non-idle conditions and may include the smallestair charge possible for avoiding engine misfire.

Upon confirming that the engine is operating in the fixed mode range,method 300 may proceed to 312 to determine if the engine is operating ina VDE mode. For example, determining engine operation in the VDE modemay include determining that the engine is operating with one or morecylinders of a given engine bank deactivated while engine cylinders of aremaining engine bank are active. In one example, the engine may beoperated in the VDE mode responsive to driver torque demand being lowerthan a threshold demand. Upon confirming that the engine is operating inthe VDE mode with one or more cylinders deactivated, method 300 mayproceed to 314. At 314 the method may include transitioning to, orcontinuing to operate in, a VDE mode fixed LP-EGR schedule. In oneexample, operating the engine in a VDE mode fixed LP-EGR schedule mayinclude delivering LP-EGR at a higher fixed percentage (relative tointake air). The higher fixed percentage may be based on a minimumengine load that the engine may operate at when in the VDE mode. Inaddition, the higher fixed percentage may be based on a higher averagecylinder load of active firing cylinders when the engine is in the VDEmode. Details of engine operation in the VDE mode fixed LP-EGR scheduleare elaborated with reference to FIG. 4.

If the engine is not operating in the VDE mode (that is, if allcylinders are active), method 300 may proceed to 316 to transition to,or to continue to operate with a normal mode fixed LP-EGR schedule. Inone example, operating engine in a normal mode fixed LP-EGR schedule mayinclude delivering LP-EGR at a lower fixed percentage. The lower fixedpercentage may be based on a minimum cylinder load that the engine mayoperate in the non-VDE mode. In addition, the lower fixed percentage maybe based on a lower average cylinder load of firing cylinders when theengine is in the non-VDE mode. As such, the minimum cylinder load thatthe engine encounters during the VDE mode may be greater than theminimum cylinder load during the non-VDE mode due to fewer activecylinders combusting to meet the torque demand. Consequently, the EGRpercentage provided during the VDE mode (that is, the higher EGRpercentage) may be greater than the EGR percentage provided during thenon-VDE mode (that is, the lower EGR percentage). Details of engineoperation in the normal mode fixed LP-EGR schedule will be furtherelaborated with reference to FIG. 4. In some examples, the normal modefixed EGR schedule may be utilized during conditions when enablement ofVDE is not possible, such as when driver torque demand is higher than athreshold demand or an engine oil temperature is lower than a thresholdtemperature.

It will be appreciated that while the minimum cylinder load is utilizedin the given example, in alternate examples, air load per enginecombustion may be utilized. For example, the higher fixed percentage EGR(with respect to intake air flow) may be based on a minimum air loadduring VDE mode, and the lower fixed percentage EGR (with respect tointake air flow) may be based on a minimum air load during non-VDE mode.

Returning to 310, if the engine is not operating with speed and load inthe fixed range, method 300 may proceed to 314 to determine if theengine is operating at idle. Idle conditions may include engine speed,load, and vehicle speed being below a threshold, as well as brake pedalposition past a threshold, transmission in park, etc. If it isdetermined that the engine is operating at idle, method 300 may proceedto 316 to transition to, or continue operating in, an idle LP-EGRschedule. Operating in the idle LP-EGR schedule may include, in oneexample, delivering an idle EGR percentage rate of fresh airflow, theidle rate being lower than the EGR percentage maintained during each ofthe normal mode fixed LP-EGR schedule and the VDE mode fixed LP-EGRschedule. In another example, operating in the idle LP-EGR schedule mayinclude blocking airflow through the LP-EGR system, and thus the LP-EGRvalve may be closed while in the idle mode. Operating in the idle LP-EGRschedule may also include adjusting a throttle and adjusting a sparktiming. For example, throttle opening may be reduced and spark timingmay be advanced during the idle mode.

If the engine is not operating at idle, method 300 may proceed to 318 totransition to, or continue to operate in, a variable LP-EGR schedule.The variable LP-EGR schedule may be enabled at engine speed and engineloads outside the fixed schedule range, and in some examples maycomprise all engine loads above mid-load (e.g., above 50% engine load)and all engine speeds above a threshold, such as above 3500 RPM.Enabling the variable EGR mode includes ramping the LP-EGR valve openand adjusting the valve so that the desired EGR percentage rate of freshairflow is maintained. The LP-EGR valve may be adjusted to deliver anamount of LP-EGR such that the EGR percentage rate of fresh airflow inthe intake passage is varied based on engine speed and engine loadchanges. Enabling the variable LP-EGR schedule may also includeadjusting the throttle and adjusting the spark timing.

Thus, method 300 provides for determining the desired amount of HP-EGRto deliver to the intake based on engine speed and load, and furtherprovides for determining which LP-EGR schedule to operate in. The lowerfixed schedule may optimize delivery of LP-EGR during engine operationin a non-VDE mode by maintaining a lower fixed EGR percentage rate offresh airflow, that is, maintain lower fixed EGR percentage of freshairflow within the total airflow, which includes the EGR and freshairflow. The higher fixed schedule may optimize delivery of EGR in a VDEmode by maintaining a higher fixed percentage of fresh air flow. Bydelivering fixed EGR percentage in two schedules based on the mode ofengine operation, effectiveness of LP-EGR during different modes ofengine operation may be improved.

In some embodiments, the control of the LP-EGR system is maintainedindependently of the HP-EGR system. Thus, under certain operatingconditions, the HP-EGR rate may be varied as speed and/or load change,while the LP-EGR percentage is fixed, even as load changes. Under otherconditions, such as during a transition from the fixed to variable mode,the HP-EGR rate may be adjusted in response to the transition.

Turning to FIG. 4, a method 400 for adjusting an LP-EGR schedule duringengine operation in a speed-load area for a fixed LP-EGR schedule isshown. As discussed at FIG. 3, a fixed rate LP-EGR schedule may beutilized during engine operating conditions at engine loads frommid-load down to minimum engine load, and/or engine speeds lower than athreshold, such as 3500 RPM. Further, when operating in the fixedschedule range, a VDE mode LP-EGR fixed schedule may be utilized duringa VDE mode, while a normal mode LP-EGR fixed schedule may be utilizedduring a non-VDE mode. In the examples illustrated herein, the VDE modeLP-EGR schedule may also be referred to as a higher fixed schedule, andthe normal mode LP-EGR fixed schedule may also be referred to as a lowerfixed schedule. The method of FIG. 4 may be stored as executableinstructions in non-transitory memory of controller 12 shown in FIGS.1-2 and carried out by the controller in combination with the varioussensor, actuators, and engine components illustrated in FIGS. 1-2.

Method 400 may include, at 402, determining engine operating conditions.Engine operating conditions may include engine speed, engine load,vehicle speed, engine temperature, exhaust catalyst temperature,manifold pressure (MAP), manifold air flow (MAF), barometric pressureetc., measured and/or estimated from sensors including throttle positionsensor, pedal position sensor, etc. As such, in the given example, theengine may be operating at speed and load conditions in the fixedschedule range for LP-EGR, wherein the fixed schedule range includes allengine loads from mid load down to minimum engine load, and/or enginespeeds lower than a threshold, such as 3500 RPM.

At 404, based on the estimated operating conditions, method 400 mayinclude determining an engine mode of operation (e.g., VDE, or non-VDE).For example, if the torque demand is low, the engine may be operating ina VDE mode with one or more cylinders deactivated while the torquedemand is met by the remaining active cylinders. In comparison, if thetorque demand is high, the engine may be operating in a non-VDE modewith all the cylinders active.

At 406, the LP-EGR schedule may be determined based on the estimatedoperating conditions and the mode of engine operation (VDE or non-VDE).As discussed above, in the given example, the engine may be operating atspeed and load conditions in the fixed mode range for LP-EGR. During theVDE mode of operation when one or more cylinders deactivated, operatingEGR in the fixed schedule may include providing LP-EGR at the higherfixed schedule; and during non-VDE mode of engine operation when allcylinders are active and combusting, operating EGR in the fixed schedulemay include providing LP-EGR at the lower fixed schedule. In oneexample, the higher fixed schedule may be based on a first minimumcylinder load in the VDE mode, and the lower fixed schedule may be basedon a second minimum cylinder load in the non-VDE mode, wherein the firstminimum cylinder load in the VDE mode is greater than the first minimumcylinder load in the non-VDE mode. In another example, the higher fixedschedule may include providing EGR at 20% with respect to intake airflow, and the lower fixed schedule may include providing EGR at 15% withrespect to intake air flow.

Next, at 408, method 400 may include confirming if the engine isoperating in a VDE mode. In one example, the VDE mode may be confirmedwhen one or more cylinders are deactivated. The one or more cylindersmay be deactivated in response to a torque demand less than a thresholddemand, for example. Upon confirming engine operation in the VDE mode,method 400 may proceed to 412.

At 412, method 400 may include providing EGR at the higher fixedschedule which includes providing LP-EGR at a higher fixed percentage.Providing LP-EGR at the higher fixed percentage may include adjustingthe valve so that a fixed EGR percentage rate of fresh airflow ismaintained. The airflow may be measured in the intake passage downstreamof a point where LP-EGR and fresh air mix and upstream of a charge aircooler, such as CAC 154. The percentage EGR of fresh airflow may bedetermined by an oxygen sensor, such as sensor 145. The LP-EGR valve maybe adjusted to deliver an amount of LP-EGR such that the EGR percentageof fresh airflow in the intake passage is maintained at the higher fixedpercentage rate, regardless of engine speed and load changes, while theengine is operating in the fixed schedule range and in the VDE mode. Insome examples, the higher fixed EGR percentage rate of fresh airflow maybe a total airflow wherein 80% of the airflow comprises fresh air while20% comprises EGR. Any suitable fresh airflow percentage that maintainsfuel economy, emissions, combustion stability, and power output atdesired levels during the VDE mode may be used.

Returning to 408, if engine operation in the VDE mode is not confirmed,method 400 may proceed to 410. For example, if the engine is notoperating in the VDE mode, the engine may be operating in a non-VDEmode. Accordingly, at 410, method 400 may include providing EGR at anormal mode flat schedule. For example, when engine is operating in thenon-VDE mode, the EGR tolerance level for the engine may be lower.Consequently, EGR may be provided at a second lower fixed schedule whichmay be lower than the higher fixed schedule. Providing LP-EGR at thelower fixed percentage may include adjusting the LP-EGR valve to deliverLP-EGR such that the EGR percentage of fresh airflow in the intakepassage is maintained at the lower fixed percentage rate, regardless ofengine speed and load changes, while the engine is operating in thefixed schedule range and in the VDE mode. In one example, during non-VDEmode of engine operation, LP-EGR may be provided at 15% with respect tointake air flow.

Returning to 412, upon providing LP-EGR at the higher VDE mode fixedschedule, method 400 may proceed to 414. At 414, method 400 may includeconfirming if a tip-out event has occurred. For example, confirming atip-out event may include determining if the operator has released theaccelerator pedal. In one example, in response to the tip-out operation,torque demand may drop from a higher torque demand to a lower torquedemand below the threshold demand. In another example, in response tothe tip-out operation, the torque demand may drop from the higher torquedemand to a minimum torque demand. If it is determined that a tip-outoperation has occurred, method 400 may proceed to 415.

At 415, upon determining the tip-out operation, method 400 may includedetermining if the engine may be transitioned out of the VDE mode to thenon-VDE mode (in order to reduce NVH issues, for example). If the answeris NO, method 400 may proceed to 419. If the answer at 415 is YES,method 400 may proceed to 420.

At 419, upon confirming that engine transition out of the VDE mode isnot required, method 400 may include continuing providing EGR at the VDEmode flat schedule which includes providing LP-EGR at a higher fixedpercentage with respect to intake air flow.

At 420, upon confirming engine transition out of the VDE mode (inresponse to the tip-out operation), method 400 may include determiningif a current engine load is greater than a threshold engine load. If thecurrent load is not greater than the threshold load, method 400 mayproceed to 422 to change EGR schedule from the higher fixed schedule(that is, VDE mode fixed LP-EGR schedule) to the lower fixed schedule(that is, normal mode fixed LP-EGR schedule) while continuing engineoperation in the VDE mode. For example, in response to the tip-outoperation, such as, tip-out to idle conditions or lower engine loadconditions, the engine operation may transition out of VDE in order toreduce NVH issues. Since an EGR tolerance level of the engine is basedon cylinder load, during the tip-out, as the engine load changes fromhigher load to lower load, the EGR tolerance level of the engine maygradually decrease. Due to large air induction volume between thecompressor and the intake manifold of the LP-EGR system, during tip-outconditions when the engine load is lower than the threshold load, if theengine is transitioned out of the VDE mode (e.g. to idle or to lowerload) before the LP-EGR is fully purged from the intake manifold, theengine may operate with higher EGR than tolerable upon transitioning outof VDE. As a result, engine misfires may occur. Therefore, in responseto a tip-out event, when the engine is operating in the VDE mode withengine load less than the threshold engine load, the EGR schedule may betransitioned from the higher fixed schedule to lower fixed schedulewhile continuing to operate the engine in the VDE mode. The engine maythen be transitioned out of the VDE mode only after the EGR rate hasdecreased from the higher fixed percentage to the lower fixedpercentage, that is, to an EGR level that the active cylinders cantolerate. As an example, EGR schedule may be transitioned from a 20%fixed EGR rate (VDE schedule) with respect to intake air flow to a 15%fixed EGR rate (normal schedule) with respect to intake air flow whilecontinuing operation in the VDE mode, and then when the EGR rate hasbeen purged to 15%, the deactivated cylinders may be reactivated and theengine may be transitioned out of the VDE mode. Decreasing EGR schedulefrom the higher fixed percentage to the lower fixed percentage mayinclude adjusting the EGR valve (decreasing an opening of the EGR valve)to deliver the lower fixed percentage.

In one example, when operating in a VDE mode, during tip-out conditions(such as tip-out conditions triggering engine transition out of VDE) toan engine load below a threshold engine load, engine transition out ofVDE may be delayed until the EGR percentage of intake air flow isreduced from a first higher percentage to a second lower percentage, thelower percentage utilized during engine operation in the non-VDE mode.In comparison, when operating in the VDE mode, during tip-out conditions(such as tip-out conditions triggering engine transition out of VDE) toan engine load above the threshold engine load, engine transition out ofVDE may be performed while (that is, concurrent to) the EGR percentageof intake air flow is reduced from a first higher percentage to a secondlower percentage, the lower percentage utilized during engine operationin the non-VDE mode.

In another example, when operating in the VDE mode, in response to atip-out occurring at a first load below a threshold to a second loadbelow the threshold, engine transition out of VDE may be delayed untilthe EGR percentage of intake air flow is reduced from a first higherpercentage to a second lower percentage, the lower percentage utilizedduring engine operation in the non-VDE mode. In comparison, in responseto a tip-out occurring at a first load above the threshold load to asecond load below the threshold load, engine transition out of VDE maybe performed while the EGR percentage of intake air flow is reduced froma first higher percentage to a second lower percentage, the lowerpercentage utilized during engine operation in the non-VDE mode

Next at 426, method 400 may include determining if the LP-EGR percentagehas reached the lower fixed schedule percentage. In one example, it maybe determined if the EGR scheduling has decreased from 20% LP-EGR to 15%LP-EGR. If the EGR percentage of intake air flow has reached the lowerfixed schedule percentage, method 400 may proceed to 430 to transitionthe engine out of the VDE mode. For example, in response to the tip-outoperation when operating in VDE, upon determining that the LP-EGR haspurged to normal mode EGR levels (that is, lower fixed percentage), theengine may begin to transition out of the VDE mode. Transitioning out ofthe VDE mode includes reactivating the deactivated cylinders by resumingfuel, valve, and spark operation so that all the cylinders are activeand combusting. If the EGR percentage has not reached the normal modefixed schedule percentage, method 400 may proceed to 428 to delaytransitioning the engine out of the VDE mode until the EGR percentagedecreases to the normal mode EGR level.

Returning to 420, in response to the tip-out operation, if it isdetermined that the current engine load is greater than the threshold,method 400 may proceed to 424 to transition EGR from the higher VDE modefixed LP-EGR schedule to the lower normal mode fixed LP-EGR schedulewhile transitioning engine operation out of VDE mode. In other words,transition of engine operation out of VDE mode may not be delayed ifduring the tip-out the engine is operating at an engine load greaterthan the threshold. For example, during the tip-out, since the currentload is greater than the threshold load, the engine may be able totolerate higher EGR levels. Therefore, the transition of engineoperation out of VDE mode and transition of EGR schedule from the VDEmode fixed LP-EGR schedule to the normal mode fixed LP-EGR schedule mayoccur simultaneously. That is, transition of engine operation out of VDEmode may not be delayed until EGR is purged to normal mode level.

In this way, during engine operation in the VDE mode, in response to thetip-out operation occurring when the current load is greater thanthreshold, EGR schedule may be transitioned from the higher fixed VDEmode schedule to the lower fixed VDE mode schedule while transitioningthe engine out of the VDE mode. However, if the tip-out occurs when thecurrent engine load is less than the threshold load, EGR schedule may betransitioned from the higher fixed VDE mode schedule to the lower fixednormal mode schedule, and the engine operation may be transitioned outof the VDE mode only when the EGR level reached the normal mode fixedschedule rate. By delaying transition of engine operation from the VDEmode during tip-out when the current load is less than the thresholdload, EGR tolerance of the engine during transient conditions may beimproved, and consequently, engine misfires may be reduced.

Returning to 414, if the tip-out condition is not confirmed, method 400may proceed to 416 to confirm if a tip-in to a higher engine load isdetected. The higher load may be greater than a higher engine loadthreshold. If a tip-in is confirmed, method 400 may proceed to 417 toconfirm if the engine may be transitioned out of the VDE mode. If yes,method 400 may proceed to 418 to transition engine operation out of VDEmode while transitioning EGR schedule based on engine speed and engineload conditions. If the answer at 417 is NO, method 400 may proceed to419 to continue providing EGR at VDE mode flat schedule which includesproviding LP-EGR at the higher fixed percentage with respect to intakeair flow as discussed above.

In one example, during the tip-in, the engine speed and engine load mayincrease such that the engine may be operating outside the speed andload range for fixed mode EGR schedule, and in some embodiments mayinclude all engine speeds above a threshold, such as 3500 RPMs, and allengine loads above mid-load (e.g., above 50% load). Consequently, inresponse to the tip-in, EGR may be transitioned from the fixed scheduleto a variable schedule. Transitioning to the variable schedule from thefixed schedule may include adjusting the LP-EGR valve to deliver adesired EGR percentage rate of fresh airflow such that the EGRpercentage rate of fresh airflow in the intake passage is varied basedon engine speed and load changes. For example, transitioning to thevariable mode may include adjusting the valve to increase the LP-EGRrate. The transition may also include opening the throttle and advancingspark timing.

In another example, during the tip-in, the engine speed and load mayincrease. However, the engine may continue to operate in the speed andload range for fixed LP-EGR schedule. Consequently, in response to thetip-in, EGR may be transitioned from the higher fixed schedule (VDE) tothe lower fixed mode normal schedule (non-VDE). As such, during tip-into higher engine load, EGR tolerance may be higher, and therefore, EGRmay transition to the normal mode fixed schedule from the VDE mode fixedschedule while transitioning engine operation out of the VDE mode. Forexample, in response to the tip-in, EGR may be transitioned from 20% to15% while transitioning engine out of the VDE mode.

In some examples, in response to the tip-in to higher engine load,engine operation may transition out of the VDE mode only when anexpected engine load in the normal (non-VDE) mode is greater than athreshold VDE engine load. For example, the threshold VDE engine loadmay be a minimum engine load in the VDE mode.

In this way, during tip-in conditions, LP-EGR may be provided based onengine speed-load conditions and the mode of engine operation to ensureoptimal amount of EGR during transient engine operations.

Referring now to FIGS. 5A and 5B, example graphs illustrating maps forexample LP-EGR schedule operating regions are shown. Specifically, FIG.5A shows a 2-dimensional map 500 a indicating operating regions for afixed LP-EGR schedule and a variable LP-EGR schedule based on enginespeed and load conditions. Map 500 a shows engine load (which may be anair charge and/or airflow) on the Y-axis and engine speed on the X-axis.While this example illustrates speed and load as example parameters,various other parameters such as torque, transmission speed, enginecoolant temperature, vehicle speed, etc., may be used.

In the given example, a fixed LP-EGR schedule operating region isdepicted by a rectangular window 502. As discussed at FIG. 3, the fixedschedule range may include all engine loads from mid load down tominimum engine load, and/or engine speeds lower than a threshold, suchas 3500 RPMs. A variable LP-EGR schedule operating region is depicted atengine speed and load conditions outside the fixed schedule range, andin some example may include all engine loads above mid-load (e.g., above50% engine load) and all engine speeds above a threshold, such as 3500RPMs.

During engine operating conditions in the fixed schedule range, LP-EGRmay be provided at the fixed LP-EGR schedule, wherein LP-EGR percentageof intake air flow remains constant over the fixed speed and load range.Further, providing the fixed LP-EGR schedule may include providing ahigher fixed EGR flow percentage or a lower fixed flow percentage basedon an engine mode of operation (e.g., VDE or non-VDE). Details ofoperating regions within the fixed LP-EGR mode will be furtherelaborated with respect to FIG. 5B.

During engine operating conditions in the variable schedule range,LP-EGR may be provided at the variable EGR schedule, wherein the LP-EGRpercentage of intake air flow is varied based on engine speed and loadchanges.

Further, based on speed and load of engine operation, LP-EGR schedulemay be transitioned from the fixed mode range to the variable moderange, and vice-versa. As an example, during a tip-in to higher engineload conditions, engine operation may vary from the fixed schedule rangeto the variable schedule range (at a condition of higher speed and loadcompared to the fixed mode). As a result of increased engine speed andload from the fixed range to the variable range, the EGR schedule may betransitioned from the fixed mode EGR schedule (providing fixed EGRpercentage with respect to intake air flow) to the variable mode EGRrate (providing variable EGR percentage with respect to intake air flowbased on engine speed and load).

In this manner, an engine such as engine 10 at FIGS. 1 and 2 may beconfigured to operate at different EGR schedules based on engineoperating conditions. As the operating conditions of the engine varywith engine operation, the engine may be controlled by the controlsystem (e.g. controller 12 at FIGS. 1 and 2) to transition betweenvarious EGR schedules. Further, these transitions may be performed byadjusting one or more operating conditions such as EGR valve opening,spark timing, and/or throttle opening.

Now turning to FIG. 5B, it shows a 3-dimensional map 500 b for the fixedmode LP-EGR schedule including a normal mode fixed LP-EGR schedule and aVDE mode fixed LP-EGR schedule. Map 500 b shows engine load on theY-axis, engine speed on the X-axis and LP-EGR percentage on the Z-axis.While this example illustrates speed and load as example parameters forX-axis and Y-axis respectively, various other parameters such as torque,transmission speed, engine coolant temperature, vehicle speed, etc., maybe used while maintaining LP-EGR as the parameter for Z-axis.

During engine operation in the fixed mode operating range, LP-EGR may beprovided at the normal fixed EGR schedule or the VDE fixed EGR schedulebased on an engine operating mode. For example, when the engine isoperating in a VDE mode with one or more cylinders deactivated, LP-EGRmay be provided at the VDE mode fixed schedule. The VDE mode fixedschedule EGR rate is based on a minimum cylinder load during VDE mode.In the given example, the operating region for the higher fixed mode VDEis depicted by rectangular box 504, and EGR percentage at the VDE fixedmode is 20% of air flow in the intake manifold. Further, when the engineis operating in a normal mode (that is, non-VDE mode) with all thecylinders active and combusting, LP-EGR may be provided at the normalmode fixed schedule. The normal mode fixed schedule EGR rate is based ona minimum cylinder load during normal mode. In the given example, theoperating region for the normal fixed mode is depicted by rectangularbox 506, and the EGR percentage at the normal fixed mode is 15% of airflow in the intake manifold. As such the minimum cylinder load duringthe VDE mode is greater than the minimum cylinder load during the normalload. Consequently, EGR percentage during the VDE mode is greater thanthe EGR percentage during the normal (non-VDE) mode.

It will be appreciated that while in the examples discussed herein,minimum cylinder load is utilized, in alternate examples, air load perengine combustion may be utilized. For example, the VDE mode fixedschedule EGR rate may be based on a minimum air load during VDE mode,and the normal mode fixed schedule EGR rate is based on a minimum airload during normal mode.

Further, when the engine operation demands transition from the VDE modeof engine operation to the normal (non-VDE) mode of engine operation,based on the engine load at the time of engine operation transitionrequest, the engine operation and the EGR scheduling may be transitionedsimultaneously or the engine operation may transition after the EGRscheduling is transitioned. For example, when operating the engine inthe VDE mode below a threshold engine load 512, in response to an engineoperation transition request from the VDE mode to the non-VDE mode suchas during a tip-out (in order to reduce NVH concerns), the engineoperation transition out of the VDE mode may be delayed until the EGR ispurged from the VDE mode fixed LP-EGR schedule to the normal mode fixedLP-EGR schedule. That is, during engine operation in the speed loadregion below threshold load 512, EGR may be first allowed to decreasefrom 20% to 15%. Upon the EGR percentage reaching 15%, the transition ofengine operation out of the VDE mode may commence.

However, when operating the engine in the VDE mode above the thresholdengine load, in response to the tip-out condition, engine operation maytransition out of the VDE mode while transitioning the EGR schedule fromthe VDE mode fixed LP-EGR schedule to the normal mode fixed LP-EGRschedule.

In this way, by delaying transition of engine operation from the VDEmode to the non-VDE mode, in response to a tip-out confirmation below athreshold engine load, EGR may be purged to lower levels prior to enginetransition. Consequently, excess dilution during transient conditionssuch as tip-out, may be reduced, and the engine cylinder may not beoperated with a higher EGR level than it can tolerate. As a result,engine misfires due to excess engine dilution may be reduced.

Turning to FIG. 6, it shows operating sequence 600 depicting an examplefixed LP-EGR scheduling for a VDE engine based on a mode of engineoperation. FIG. 6 illustrates example pedal position at plot 602, engineload at plot 604, total air charge at plot 606, LP-EGR flow rate at plot610, LP-EGR percentage at plot 612, and engine mode at plot 618. Thesequence of events in FIG. 6 may be provided by executing instructionsin the system of FIGS. 1-2 according to the method of FIG. 4. Verticalmarkers at times t0-t7 represent times of interest during the sequence.In all the plots discussed below, the X axis represents time and timeincreases from the left side of each plot to the right side of eachplot.

The first plot from the top of FIG. 6 represents accelerator pedalposition versus time. The Y axis represents accelerator pedal positionand a depression of the accelerator pedal increases in the direction ofthe Y axis arrow.

The second plot from the top of FIG. 6 represents engine load versustime. The Y axis represents engine load and the load increases in thedirection of Y axis arrow. Horizontal line 606 represents a thresholdengine load. The threshold engine load may be based on the EGR purgetime at that engine air flow rate. The threshold engine load may becalibrated or modeled to ensure that the EGR rate in the intake manifoldmay be adjusted from the higher schedule to the lower schedule beforethe cylinder load can decrease to a cylinder load that will experienceslow burns or misfires at the higher EGR schedule.

In one example, the threshold engine load may be based on a minimum loadfor EGR tolerance resulting in acceptable combustion stability.

The third plot from the top of FIG. 6 represents total air charge versustime. The Y axis represents total air charge and the total air chargeincreases in the direction of the Y axis arrow. Total air chargeincludes all air flowing through the LP-EGR system and includes bothfresh airflow from the intake and EGR.

The fourth plot from the top of FIG. 6 represents LP-EGR flow rateversus time. The Y axis represents LP-EGR flow rate and the LP-EGR flowrate increases in the direction of the Y axis arrow. The LP-EGR flowrate may be the rate of LP-EGR mass flowing through the system.

The fifth plot from the top of FIG. 6 represents LP-EGR percentageversus time. The Y axis represents LP-EGR percentage, and the LP-EGRpercentage increases in the direction of Y axis arrow. The LP-EGRpercentage includes the relative amount of LP-EGR flow comprising thetotal airflow, that is, the percentage of LP-EGR within the total aircharge. Horizontal line 614 represents the LP-EGR percentage during aVDE fixed mode schedule, and horizontal line 616 represents the LP-EGRpercentage during a normal fixed mode schedule. In one example, theLP-EGR percentage when operating in the VDE mode fixed schedule may be20% with respect to fresh intake air flow and the LP-EGR percentage whenoperating in the normal mode fixed schedule may be 15% with respect tointake air flow. As such, the LP-EGR percentage in the VDE mode fixedschedule may be greater than the LP-EGR percentage in the normal modefixed schedule.

The sixth plot from the top of FIG. 6 represents engine mode versustime. The Y axis represents an engine mode of operation and the enginemay operate in a VDE mode when one or more cylinders of the engine aredeactivated or a non-VDE mode when all the cylinders are active andcombusting.

At time t0, and between times t0 and t1, the engine may be operating ina low to mid-load range in steady state conditions with pedal position602 maintained at a substantially constant position. Further, the enginemay be operating in the VDE mode 608 with one or more cylindersdeactivated, and therefore, LP-EGR may be provided at a higher VDE modefixed schedule percentage (614).

At time t1, a vehicle operator may depress the accelerator pedal andinitiate a “tip-in” event. As a result, a throttle (not shown) may openmore, and the engine load, the total air flow 608, and the EGR flow rate610 may increase. In response to the tip-in, the engine operation maytransition from the VDE mode to the non-VDE mode and at the same time,the EGR schedule may also transition from VDE mode fixed schedule (e.g.,20% EGR) to normal mode fixed schedule (e.g., 15% EGR). Even though itmay take a duration of time for the EGR percentage in the intakemanifold to decrease from the VDE mode fixed schedule percentage tonormal mode fixed schedule percentage, due to higher cylinder loadconditions during the tip-in, the engine may tolerate higher EGRamounts. Hence, delaying transition of engine operation from the VDEmode to the non-VDE mode may not be required and the cylinders may bereactivated while EGR is ramped down.

In some examples, in response to the tip-in triggering transition out ofthe VDE mode, if an expected engine load in the non-VDE mode is lessthan or equal to a minimum VDE engine load threshold, engine transitionfrom the VDE mode to the non-VDE mode may be delayed until the expectedload exceeds the minimum VDE load threshold. As such, the minimum VDEload threshold may be based on a minimum engine load when operating theengine in the VDE mode.

Next, at time t2, the vehicle operator may release the accelerator pedaland initiate a “tip-out” event. As a result, the engine load and thetotal air charge may decrease to the initial amounts (that is, prior tot1). Further, in response to decrease in engine load, engine operationmay transition from the non-VDE mode to VDE mode, and the EGR schedulemay transition from the normal mode fixed schedule to the VDE mode fixedschedule. That is, the EGR percentage of intake air flow may increase tothe higher VDE mode fixed percentage. When the engine load decreasesduring the tip-out, the EGR flow rate and the total air charge may alsodecrease.

At time t3, a second tip-out event may occur and the engine load maydecrease. Further, the tip-out may occur when the engine load is greaterthan the threshold engine load. In response to the tip-out occurringwhen the engine load is greater than the threshold, the EGR schedule maytransition from the VDE mode to the normal mode while transitioning theengine out of the VDE mode. As a result, EGR percentage may decrease tothe lower normal mode fixed percentage (616), and the EGR flow rate mayalso decrease as the engine load and the total air charge decreases.Transitioning the engine out of the VDE mode in response to the tip-outmay reduce NVH issues. Between t3 and t4, upon completion of thetip-out, the engine may be operating in the non-VDE mode with the LP-EGRprovided at the normal mode schedule.

Next, at t4, a second tip-in event may be initiated. In response to thetip-in, the engine load and the total air charge may increase, and theengine may continue to operate in the non-VDE mode. Therefore, the EGRmay continue to be provided at the lower normal fixed mode schedule.

At t5, a third tip-out event may occur resulting in engine operationtransition from the non-VDE mode to the VDE mode, and EGR scheduletransition from the normal mode fixed schedule to the VDE mode fixedschedule. Between t5 and t6, the engine may continue to operate in theVDE mode and the EGR may continue to be provided at the VDE mode fixedschedule.

At t6, a third tip-out event may occur, and as a result, the engine loadmay decrease. Further, the tip-out event may be initiated when theengine load is below the threshold engine load. In response to thetip-out occurring while operating in a VDE mode, it may be desirable totransition the engine out of the VDE mode in order to reduce NVH issues,and consequently, it may be desirable to transition the LP-EGR schedulefrom the higher VDE mode fixed percentage schedule to the lower normalmode fixed percentage schedule. However, due to large induction volumeof the intake system, there may be delay in purging LP-EGR to the lowernormal mode percentage. As a result, the engine operation may transitionfrom the VDE mode to the non-VDE mode before the LP-EGR decreases tonormal mode percentage. As such, when operating in the non-VDE mode, EGRtolerance for the engine is lower, and if the engine transitions to thenon-VDE prior to the EGR levels reaching the normal mode level, excessdilution of intake air can occur leading to engine misfires. Therefore,when the engine is in a VDE mode, in response to the tip-out occurringat the engine load below the threshold, engine transition to the non-VDEmode may be delayed for a duration d1 (between t6 and t7) until theLP-EGR is purged from the VDE mode fixed percentage to the normal modefixed percentage. That is, when operating the engine in a VDE mode, inresponse to the tip-out occurring at the engine load below the thresholdengine load, the engine may be transitioned out of the VDE mode onlywhen the LP-EGR decreases to the normal mode levels. Therefore, at t6,in response to the tip-out, engine transition out of VDE mode may bedelayed until t7 when the EGR percentage decreases to normal mode fixedpercentage of intake air flow.

As such, delivering a desired EGR percentage rate of fresh air (VDE modeschedule percentage or normal mode schedule percentage) includescoordinating adjustment of the LP-EGR valve and the throttle usingfeed-forward control, and includes feedback adjustment to maintain thepercentage based on the EGR rate measurement from, e.g., an intakeoxygen sensor. This includes changing both the intake airflow rate andthe EGR flow rate in a coordinated way such that the EGR percentage ismaintained as desired. As errors in the control may be present,maintaining the desired EGR percentage may include some small variationin the EGR percentage, e.g., 1-2% variation. During the fixed mode, whenairflow increases, such as during the tip-in event at t1, the LP-EGRvalve is adjusted to provide a corresponding increase in EGR flow tomaintain the fixed EGR percentage.

In this way, EGR may be delivered according to a flat schedule to reduceEGR errors and improve the range of EGR delivery. The flat schedule maybe further adjusted based on the operating mode of an engine withselectively deactivatable cylinders to account for the higher minimumcylinder loads applied when selected cylinders are deactivated and thelower minimum cylinder loads applied when all cylinders are active. Byapplying the schedules based on operating conditions, at each mode, theengine may be operated with an EGR schedule that the engine cantolerate. As such, this improves EGR delivery and EGR tolerance, whilereducing the occurrence of misfire events.

It will be appreciated that while in some examples discussed herein, inresponse to tip-out occurring while operating the engine in the VDEmode, the engine operation may be transitioned out of the VDE mode (inorder to reduce NVH issues, for example), in alternate examples, inresponse to tip-out conditions, the engine may continue to operate inthe VDE mode. While in the VDE mode, EGR may be provided at a higherfixed percentage with respect to intake air flow.

In one example, the sequence of FIG. 6 illustrates a method for anengine, comprising: during a first tip-out, when operating in a VDE modeand above a threshold load, reactivating engine cylinders whiletransitioning from a higher EGR schedule to a lower EGR schedule; andduring a second tip-out, when operating in the VDE mode and below thethreshold load, reactivating engine cylinders after transitioning fromthe higher EGR schedule to the lower EGR schedule. Further, the methodincludes wherein the higher EGR schedule is based on a first EGRtolerance limit during engine operation in the VDE mode, and wherein thelower EGR schedule is based on a second EGR tolerance limit duringengine operation a non-VDE mode, the first EGR tolerance limit greaterthan the second EGR tolerance limit. The method further includes whereinengine operation in the VDE mode includes operating with a total numberof deactivated engine cylinders while delivering a higher fixed LP-EGRrelative to intake airflow at the higher EGR schedule, and whereinengine operation in the non-VDE mode includes operating with all enginecylinders active while delivering a lower fixed LP-EGR relative tointake airflow at the lower EGR schedule. Still further, in someexamples the method includes retarding a spark timing when all thecylinders are reactivated.

Note that the example control and estimation routines included hereincan be used with various engine and/or vehicle system configurations.The control methods and routines disclosed herein may be stored asexecutable instructions in non-transitory memory and may be carried outby the control system including the controller in combination with thevarious sensors, actuators, and other engine hardware. The specificroutines described herein may represent one or more of any number ofprocessing strategies such as event-driven, interrupt-driven,multi-tasking, multi-threading, and the like. As such, various actions,operations, and/or functions illustrated may be performed in thesequence illustrated, in parallel, or in some cases omitted. Likewise,the order of processing is not necessarily required to achieve thefeatures and advantages of the example embodiments described herein, butis provided for ease of illustration and description. One or more of theillustrated actions, operations and/or functions may be repeatedlyperformed depending on the particular strategy being used. Further, thedescribed actions, operations and/or functions may graphically representcode to be programmed into non-transitory memory of the computerreadable storage medium in the engine control system, where thedescribed actions are carried out by executing the instructions in asystem including the various engine hardware components in combinationwith the electronic controller.

It will be appreciated that the configurations and routines disclosedherein are exemplary in nature, and that these specific embodiments arenot to be considered in a limiting sense, because numerous variationsare possible. For example, the above technology can be applied to V-6,I-4, I-6, V-12, opposed 4, and other engine types. The subject matter ofthe present disclosure includes all novel and non-obvious combinationsand sub-combinations of the various systems and configurations, andother features, functions, and/or properties disclosed herein.

The following claims particularly point out certain combinations andsub-combinations regarded as novel and non-obvious. These claims mayrefer to “an” element or “a first” element or the equivalent thereof.Such claims should be understood to include incorporation of one or moresuch elements, neither requiring nor excluding two or more suchelements. Other combinations and sub-combinations of the disclosedfeatures, functions, elements, and/or properties may be claimed throughamendment of the present claims or through presentation of new claims inthis or a related application. Such claims, whether broader, narrower,equal, or different in scope to the original claims, also are regardedas included within the subject matter of the present disclosure.

1. A method for an engine, comprising: in response to a first tip-outoccurring while operating the engine below a threshold engine load withone or more cylinders deactivated and with EGR provided at a higherfixed schedule relative to intake air flow, delaying reactivation of thedeactivated cylinders until EGR has reduced from the higher fixedschedule to a lower fixed schedule relative to intake air flow.
 2. Themethod of claim 1, further comprising, in response to a second tip-outoccurring while operating the engine above the threshold engine loadwith one or more cylinders deactivated and with EGR provided at thehigher fixed schedule, reactivating the deactivated cylinders whilereducing EGR from the higher fixed schedule to the lower fixed schedule.3. The method of claim 2, further comprising, in response to a tip-inoccurring when operating the engine with one or more cylindersdeactivated and with EGR provided at the higher fixed schedule, delayingreactivation of the deactivated cylinders until an expected cylinderload with all cylinders active is greater than a first minimum cylinderload.
 4. The method of claim 3, wherein the higher fixed schedule of EGRis based on the first minimum cylinder load of the engine when the oneor more cylinders are deactivated.
 5. The method of claim 4, wherein thelower fixed schedule is based on a second minimum cylinder load of theengine when all engine cylinders are active.
 6. The method of claim 5,wherein the first minimum cylinder load is greater than the secondminimum cylinder load.
 7. The method of claim 5, wherein a first EGRrate provided at the higher fixed schedule is greater than a second EGRrate provided at the lower fixed schedule.
 8. The method of claim 7,wherein providing EGR at the lower fixed schedule includes providing thesecond EGR rate at 15% of intake airflow, and wherein providing EGR atthe higher fixed schedule includes providing the first EGR rate at 20%of intake airflow.
 9. The method of claim 1, wherein reducing EGR to thelower fixed schedule includes decreasing an opening of the EGR valve.10. The method of claim 1, wherein the EGR is provided by a low pressureEGR.
 11. An engine method, comprising: operating a low-pressure exhaustgas recirculation (LP-EGR) LP-EGR system in a first mode with a first,fixed EGR schedule based on a first minimum cylinder load present whenone or more cylinders are selectively deactivated; and operating theLP-EGR system in a second mode with a second, fixed EGR schedule basedon a second minimum cylinder load present when all cylinders are active.12. The method of claim 11, wherein the first minimum cylinder load isgreater than the second minimum cylinder load, and wherein a secondLP-EGR rate when operating at the second, fixed EGR schedule is lowerthan a first LP-EGR rate when operating at the first, fixed EGRschedule.
 13. The method of claim 12, wherein operating the engine inthe second mode includes retarding a spark timing.
 14. The method ofclaim 13, further comprising, responsive to a first condition,transitioning from the first mode to the second mode, and during thetransitioning, reactivating the deactivated cylinders after reducingLP-EGR from the first LP-EGR rate to the second LP-EGR rate.
 15. Themethod of claim 14, further comprising, responsive to a secondcondition, transitioning from the first mode to the second mode, andduring the transitioning, reactivating the deactivated cylinders whilereducing LP-EGR from the first LP-EGR rate to the second LP-EGR rate,wherein reducing LP-EGR from the first rate to the second rate includesreducing an opening of a LP-EGR valve included in the LP-EGR system. 16.The method of claim 15, wherein the first condition includes a tip-outoccurring at an engine load below a threshold load, and wherein thesecond condition includes a tip-out occurring at the engine load abovethe threshold load.
 17. The method of claim 16, wherein the firstcondition includes a tip-out to below a lower threshold load, andwherein the second condition includes a tip-in to above a higherthreshold load.
 18. A method for an engine, comprising: during a firsttip-out, when operating in a VDE mode and above a threshold load,reactivating engine cylinders while transitioning from a higher EGRschedule to a lower EGR schedule; and during a second tip-out, whenoperating in the VDE mode and below the threshold load, reactivatingengine cylinders after transitioning from the higher EGR schedule to thelower EGR schedule.
 19. The method of claim 18, wherein the higher EGRschedule is based on a first EGR tolerance limit during engine operationin the VDE mode, and wherein the lower EGR schedule is based on a secondEGR tolerance limit during engine operation a non-VDE mode, the firstEGR tolerance limit greater than the second EGR tolerance limit.
 20. Themethod of claim 19, wherein engine operation in the VDE mode includesoperating with a total number of deactivated engine cylinders whiledelivering a higher fixed LP-EGR relative to intake airflow at thehigher EGR schedule, and wherein engine operation in the non-VDE modeincludes operating with all engine cylinders active while delivering alower fixed LP-EGR relative to intake airflow at the lower EGR schedule.